shells made out of compressed light that can focus its power to a micrometre sized area.
I'm just gonna put this out there real quick, you do realize that terms like "Compressed light" and "can focus power into a micro meter sized area" are nothing more than star trek style techno babble, right? And mean absolutely nothing if the results can't be proven in canon? And seeing as there is no official canon for this universe, this whole thread is nothing more than an exerciser in futility?
Throwing out cool sounding techno babble in an effort to sound cool doesn't actually make you cool, it just makes you a blowhard, and continuing to do so even after that very same techno babble has been rejected as viable evidence for a number of different reasons, including the fact that in several cases it either makes no sense or completely goes against or contradicts other material, makes you stubborn.
Also it doesn't take rocket science to put two and two together and get the result that a galaxy spanning race of living machines, that can heal themselves, teleport over vast distances, have the ability to flay you alive with weapons that use technology that is literally stated to be almost magical in it's existence and use, have access to what is known to be the most powerful warships in the galaxy, some of their land units have ability to phase through solid objects, and last but not least they will often use what is basically mind control tech on those they're about to attack, might have just a slight advantage in this situation...
If you want a slightly more even match, then either choose an Imperial world or do what most OPs do in their vs threads, say that half the planet is under the control of the necrons and the other half is under the control of the ARM commander and before the hostilities started, he was able to setup a good base of operations as were the crons, and then let them have at each other.
Let's see how the ARM holds up:
galaxy spanning race of living machines ....ehh sort of, the ARM are genetically enhanced super humans whose minds are completely dedicated just to war and use extremely advanced nanotech and reproduce von neumann machine style. The CORE are more like "living" machines. In 4000 years of fighting they used up most of the resources in the galaxy.
heal themselves- big check for confirmed in both gameplay and fluff their nanotech is certainly quite potent.
teleport vast distances- they can teleport the commander across the galaxy instantly
flay you alive- very big check just "unarmed" simple construction units are beyond flaying you alive they can consume forests with their reclaimation power, Necrons will probably find just fighting construction units challenging. TA firepower even without the nuclear yields from fluff seem to be way above Necrons at least on the ground. Then there is the D-cannon in which there is no defense.
magitech- compressed light bullets anyone?
mind control- again TA has mind control tech as well that involves teleporting nanomachines directly into your brain or AI computer and physically rewiring it.
powerful space fleets- fluff wise yes TA has spaceships massive ones at that using just 5 starships they can evacuate a planet of billions. Though their combat abilities are unknown so we can't really debate that. TA's most powerful weapons include a machine that works by slamming planets into each other, and in the Core Contigency game a device capable of killing galaxies.
As for giant units play a level with cars and notice how small the cars are compared to Peewees.
The only thing the ARM doesn't have over the Necrons are anti-psyker/anti-chaos weaponry and wierd abilities like phasing through walls.